Electro-absorption modulator

ABSTRACT

Embodiments of the invention provide an electro-absorption modulator including an optical waveguide and a microwave waveguide. The microwave waveguide is electromagnetically coupled to the optical waveguide. The optical waveguide includes a quantum well region and a substantially sinusoidal structure. The waveguide mode of the optical waveguide is responsive to the substantially sinusoidal structure.

FIELD OF THE INVENTION

Embodiments of the invention relate to optical signal modulation, and more particularly to velocity matching in an electro-absorption modulator.

BACKGROUND OF THE INVENTION

Modulation of optical signals is commonly utilized to transfer information in modern communication systems. One method of modulation utilizes an electro-absorption modulator. One type of electro-absorption modulator includes a microwave waveguide electromagnetically coupled to an optical waveguide.

One type of optical waveguide includes a multiple-quantum well region formed of III-V compound semiconductors. The multiple quantum well region is produced by one or more quantum well layers of a low bandgap material interleaved between a corresponding number of barrier layers of a higher bandgap material. The bandgap energy of the quantum well layers is less than the bandgap energy of the barrier layers. Thus, a multiple quantum well structure composed of N quantum well layers has N+1 barrier layers.

Electrons and holes are confined in the same physical quantum well. As a result, the electrons and holes are in close proximity to each other and interact strongly to form exciton pairs. The exciton has a strong absorption similar to an atomic absorption. The electro-absorption modulator utilizes a microwave signal to change the excitonic absorption properties of the quantum well layers of the optical waveguide. The microwave signal is utilized to vary the electrical bias across the multiple quantum well layers. When biased at a first state, the external electric field across each quantum well layer results in a sharp optical absorption. When biased at a second state, the external electric field across each quantum well layer results in a lower optical absorption. Hence, the resulting modulated optical signal provided at an output of the electro-absorption modulator has an on-state and an off-state.

For optimal coupling between the microwave signal and the optical signal propagating in the electro-absorption modulator, the microwave signal should propagate along the device at the same velocity as the optical signal. However, the prior art devices are problematic in that the microwave and optical signals travel with different group velocities. Accordingly, the device efficiency and operating bandwidth is deleteriously reduced.

SUMMARY OF THE INVENTION

Embodiments of the invention provide an electro-absorption modulator including an optical waveguide and a microwave waveguide. The microwave waveguide is electromagnetically coupled to the optical waveguide. The optical waveguide includes a quantum well region and a substantially sinusoidal structure. The waveguide mode of the optical waveguide is responsive to the substantially sinusoidal structure.

Another embodiment provides a method for matching the group velocity of an optical signal to the phase velocity of the microwave signal in an electro-absorption modulator. The electro-absorption modulator includes an optical waveguide having a quantum well structure and a sinusoidal structure extending in a direction of propagation of an optical signal through the optical waveguide. The electro-absorption modulator further includes a microwave waveguide extending alongside the optical waveguide and electromagnetically coupled thereto. The operating mode of the optical waveguide and the phase velocity of the microwave signal propagating through the microwave waveguide are determined. The periodicity of the sinusoidal structure is selected such that the operating mode is displaced from a photonic bandgap by a displacement that sets the group velocity of the optical signal substantially equal to the phase velocity of the microwave signal.

Another embodiment provides a method of fabricating an electro-absorption modulator. The method includes forming an optical waveguide by a process including forming a first cladding layer over a substrate. A quantum well region is formed over the first cladding layer. A second cladding layer is formed over said quantum well region. A substantially sinusoidal structure extending in a direction of propagation of an optical signal through said quantum well region is formed. The waveguide mode of the optical waveguide is responsive to the substantially sinusoidal structure. The first cladding layer, quantum well region and second cladding layer are patterned to define a ridge structure. A first electrode is formed alongside the ridge structure and a second electrode is formed over the ridge structure.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments in accordance with the invention are illustrated by way of example and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which:

FIG. 1 shows a perspective view of an electro-absorption modulator in accordance with one embodiment of the invention.

FIG. 2 shows a perspective view of an electro-absorption modulator in accordance with another embodiment of the invention.

FIG. 3 shows a dispersion curve of the characteristic waveguide optical modes of electro-absorption modulator in accordance with one embodiment of the invention.

FIG. 4 shows a flow diagram of a method of fabricating an electro-absorption modulator in accordance with one embodiment of the invention.

FIG. 5 shows a flow diagram of a method of fabricating an electro-absorption modulator in accordance with another embodiment of the invention.

DETAILED DESCRIPTION

Reference will now be made in detail to the embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with these embodiments, it will be understood that they are not intended to limit the invention to these embodiments.

As an overview, a description of the structure of an electro-absorption modulator in accordance with the invention is set forth first. The electro-absorption modulator includes an optical waveguide having a sinusoidal structure along the direction of propagation of an optical signal. A method of operation and a method of determining the periodicity of the sinusoidal structure as a function of the operating mode and the induced bandgap is then presented. Finally, methods of fabricating the electro-absorption modulator with the desired periodicity are set forth. Embodiments of the invention advantageously provide for substantial velocity matching of the microwave and optical signals without large changes in the device geometry.

For ease of understanding, the relative position of various structures of the electro-absorption modulator is described with reference to an x, y and z coordinate system.

Structure of Electro-Absorption Modulator

Referring to FIG. 1, a perspective view of an electro-absorption modulator 100 in accordance with embodiments of the invention is shown. As depicted in FIG. 1, the electro-absorption modulator 100 includes a semiconductor substrate 110, a first cladding layer 120, a quantum well region 130, a second cladding layer 140, a first electrode 150, and a second electrode 160. The quantum well region 130 may be composed of a single quantum well layer sandwiched between two barrier layers, or a plurality of quantum well layers interleaved with a plurality of barrier layers. Also shown is an optional third electrode 155, a first contact region 170 in the semiconductor substrate 110, and a second contact region 175 in the second cladding layer 140.

In embodiments in accordance with the invention, the semiconductor substrate 110 is composed of n-doped indium phosphide (InP) or gallium arsenide (GaAs). Optionally, in embodiments in accordance with the invention, the first contact region 170 in the semiconductor substrate 110 is heavily doped with an n-type impurity. In such an embodiment, the heavily doped first contact region 170 extends approximately 0.5-2 micrometer (μm) below the surface of the semiconductor substrate 110. The heavily doped first contact region 170 of the semiconductor substrate 110 provides a low ohmic contact connecting the first electrode 150 and the first cladding layer 120.

The first cladding layer 120 is located on the surface of the semiconductor substrate 110. In embodiments in accordance with the invention, the surface 125 of the first cladding layer 120 has a substantially sinusoidal surface relief in an x-z plane. More specifically, the substantially sinusoidal surface relief in the x-direction has a periodicity, Λ, 121 of approximately 0.1-50 μm. The substantially sinusoidal surface relief has a peak-to-peak amplitude 123 in the z-direction of approximately 10-1000 nm. The first cladding layer 120 is composed of lightly n-doped indium gallium arsenide phosphide (InGaAsP) or indium gallium aluminum arsenide (InGaAlAs). The first cladding layer 120 has dimensions of approximately 1-3 μm in the z-direction, 1-4 μm in the y-direction, and 100-500 μm in the x-direction.

In embodiments in accordance with the invention, the first electrode 150 is located on the surface of the semiconductor substrate 110 along the sides of the first cladding layer 120. In the embodiment of FIG. 1, the first electrode 150 is a gold (Au) layer with dimensions of approximately 50-1000 nanometers (nm) in the z-direction, 10-30 μm in the y-direction, and 100-500 μm in the x-direction. In the example shown, a third electrode 155 is located on the surface of the semiconductor substrate 110. The first electrode 150 and third electrode 155 are located on opposite sides of the second semiconductor substrate 120.

In embodiments in accordance with the invention, the quantum well region 130 is located on the surface 125 of the first cladding layer 120. The quantum well region 130 conformally covers the surface 125 of the second semiconductor substrate layer 120. Thus, the quantum well region 130 is also formed as a sinusoidal structure having a periodicity substantially equal to that of the first cladding layer 120. The quantum well region 130 has dimensions of approximately 0.1-0.3 μm in the z-direction, 1-4 μm in the y-direction, and 100-500 μm in the x-direction.

In one embodiment in accordance with the invention, the quantum well region 130 is composed of a quantum well layer of low bandgap semiconductor material between a first and second barrier layers of a higher bandgap semiconductor material. In another embodiment in accordance with the invention, the quantum well region 130 is composed of N quantum well layers of low bandgap semiconductor material interleaved with N+1 barrier layers of a higher bandgap semiconductor material. The materials of the quantum well layers and barrier layers are formulated to have the above-stated bandgap relationship and to be absorptive at the operating wavelength of the electro-absorptive modulator 100. In embodiments in accordance with the invention, the quantum well layers are composed of indium arsenide phosphide (InAsP) or indium gallium arsenide (InGaAs), and the barrier layers are composed of indium gallium phosphide (InGaP) or indium aluminum arsenide (InAlAs).

The second cladding layer 140 conformally covers the surface 135 of the quantum well region 130. Hence, the surface of the second cladding layer 140 juxtaposed with the quantum well region 130 has a substantially sinusoidal surface relief in the x-z plane substantially equal to that of the first cladding layer 120. In one embodiment in accordance with the invention, the second cladding layer 140 is lightly p-doped indium gallium arsenide phosphide or indium gallium aluminum arsenide. Further, the second cladding layer 140 has dimensions of approximately 1-3 μm in the z-direction, 1-4 μm in the y-direction, and 100-500 μm in the x-direction.

Optionally, in embodiments in accordance with the invention, the second contact region 175 in the second cladding layer 140 is heavily doped with a p-type impurity. The impurity profile extends approximately 0.5-2 μm below the surface 145 of the second cladding layer 140. The heavily doped second contact region 175 of the second cladding layer 140 provides a low ohmic contact between the second cladding layer 140 and the second electrode 160.

The first cladding layer 120, the quantum well region 130 and the second cladding layer 140 constitute a ridge structure. The index of refraction of the first cladding layer 120, and the second cladding layer 140 are lower than that of the quantum well region 130. Therefore, the ridge structure forms an optical waveguide.

The second electrode 160 is located on the surface of the second cladding layer 140 remote from the quantum well region 130. The second electrode 160 is a gold layer, approximately 50-1000 nm thick, 1-4 μm wide and 100-500 μm long. Furthermore, the second electrode 160 and the first electrode 150 form a microwave waveguide.

Referring now to FIG. 2, a perspective view of an electro-absorption modulator 200 in accordance with embodiments of the invention is shown. As depicted in FIG. 2, the electro-absorption modulator 200 includes a semiconductor substrate 210, a first cladding layer 220, a quantum well region 230, a second cladding layer 240, a first and third electrodes 250, 255, and a second electrode 260. Also shown is an optional first contract region 270 in the semiconductor substrate 210, and a second contact region 275 in the second cladding layer 240.

The semiconductor substrate 210 is composed of n-doped indium phosphide (InP) or gallium arsenide (GaAs). Optionally, in embodiments in accordance with the invention, the first contact region 270 of the semiconductor substrate 210 is heavily doped with an n-type impurity. In such an embodiment, the heavily doped first contact region 270 extends approximately 0.5-2 μm below the surface of the semiconductor substrate 210. The heavily doped first contact region 270 of the semiconductor substrate 210 provides a low ohmic contact connecting the first and third electrodes 250, 255 and the first cladding layer 220.

The first cladding layer 220 is located on the surface of the semiconductor substrate 210. The first cladding layer 220 is composed of lightly n-doped indium gallium arsenide phosphide or indium gallium aluminum arsenide. The first cladding layer 120 is approximately 1-3 μm in the z-direction, 1-4 μm in the y-direction, and 100-500 μm in the x-direction.

In embodiments in accordance with the invention, the first and third electrodes 250, 255 are located on the surface of the semiconductor substrate 210, along opposite sides of the first cladding layer 220. In one embodiment in accordance with the invention, the first and third electrodes 250, 255 are composed of a gold (Au) layer and have dimensions of approximately 50-1000 nm in the z-direction, 10-30 μm in the y-direction, and 100-500 μm in the x-direction.

In embodiments in accordance with the invention, the quantum well region 230 is located on the surface of the first cladding layer 220. The quantum well region 230 has dimension of approximately 0.1-0.3 μm in the z-direction, 1-4 μm in the y-direction, and 100-500 μm in the x-direction. In one embodiment in accordance with the invention, the quantum well region 230 is composed of a quantum well layer of low bandgap semiconductor material between a first and second barrier layers of higher bandgap semiconductor material. In another embodiment in accordance with the invention, the quantum well region 230 is composed of N quantum well layers of low bandgap semiconductor material interleaved with N+1 barrier layers of higher bandgap semiconductor material. The materials of the quantum well layers and barrier layers are formulated to have the above-stated bandgap relationship and to be absorptive at the operating wavelength of the electro-absorptive modulator 100. In one embodiment in accordance with the invention, the quantum well layers are composed of an indium arsenide phosphide (InAsP) or an indium gallium arsenide (InGaAs), and the barrier layers are composed of a indium gallium phosphide (InGaP) or an indium aluminum arsenide (InAlAs).

The second cladding layer 240 is located on the surface of the quantum well region 230. In one embodiment in accordance with the invention, the second cladding layer 240 is lightly p-doped indium gallium arsenide phosphide (InGaAsP) or indium gallium aluminum arsenide (InGaAlAs). Furthermore, the second cladding layer 240 has dimensions of approximately 0.3-0.6 μm in the z-direction, 1-4 μm in the y-direction, and 100-500 μm in the x-direction.

In embodiments in accordance with the invention, the surface 225 of the second cladding layer 240 has a substantially sinusoidal surface relief in the x-z plane. More specifically, the substantially sinusoidal surface relief, in the x-direction, has a periodicity, Λ, 221 of approximately 0.1-10 μm. The substantially sinusoidal surface relief has a peak-to-peak amplitude 223, in the z-direction, of approximately 50-100 nm.

Optionally, in embodiments in accordance with the invention, the second contact region 275 conformally covers the surface 225 of the second cladding layer 240. The second contact region has dimensions of approximately 0.5-1000 μm in the z-direction, 1-4 μm in the y-direction, and 100-500 μm in the x-direction. The second contact region 275 is heavily doped with a p-type impurity. The heavily doped second contact region 275 provides a low ohmic contact between the second cladding layer and the second electrode 260.

The first cladding layer 220, the quantum well region 230 and the second cladding layer 240 constitute a ridge structure. The index of refraction of the first cladding layer 220, and the second cladding layer 240 are lower than that of the quantum well region 230. Therefore, the ridge structure forms an optical waveguide.

The second electrode 260 is located on the surface of the second cladding layer 240. The second electrode 260 is a gold (Au) layer, approximately 50-1000 nm in the z-direction, 1-4 μm in the y-direction, and 100-500 μm in the x-direction. Furthermore, the second electrode 260 and the first and third electrodes 250, 255 form a microwave waveguide.

Method of Operation

The index of refraction of the first cladding layer 120, 220 and the second cladding layer 140, 240 are lower than that of the quantum well region 130, 230. Therefore the first cladding layer 120, 220, the quantum well region 130, 230 and the second cladding layer 140, 240 form an optical waveguide. The first and second electrodes 150, 250 and 160, 260, respectively, form a microwave waveguide. Accordingly, the electro-absorption modulator 100, 200 includes an optical waveguide electro-magnetically coupled to a microwave waveguide.

The electro-absorption modulator 100, 200 utilizes the quantum-confined Stark effect to modulate an optical signal propagating in the optical waveguide. Moreover, the energy of the microwave signal propagating along the first and second electrodes 150, 250 and 160, 260, respectively, effects the excitons in the quantum well region 130, 230, thereby changing the absorption characteristics of the quantum well region 130, 230. Where an amplitude peak in a microwave signal occurs, absorption of the optical signal is induced by formation of exciton bonding. Thus, the optical signal can be amplitude modulated by varying the amplitude of the microwave signal applied between the first and second electrodes 150, 250 and 160, 260 respectively.

Referring to FIG. 1, in one embodiment in accordance with the invention, the optical waveguide 120, 130, 140 has a substantially sinusoidal structure in the x-direction (i.e., the direction of optical signal propagation). The sinusoidal structure of the optical wave-guide 120, 130, 140 provides velocity matching between the group velocity of an optical signal transmitted in the x-direction along the optical waveguide 120, 130, 140 and the phase velocity of a microwave signal transmitted in the x-direction along the microwave waveguide 150, 160. As is described in detail below, velocity matching is achieved by selecting the periodicity of the sinusoidal structure of the quantum well region 130.

Referring to FIG. 2, in another embodiment in accordance with the invention, the optical waveguide mode is responsive to the geometry of the entire optical waveguide 220, 230, 240. The waveguide mode is responsive to the geometry of the entire optical waveguide because the thickness of the quantum well region is approximately 0.1-0.3 μm, which is much smaller than the wavelength of the propagating optical signal (e.g., approximately 1.5 μm). A substantial portion of the energy of the optical signal propagates as the evanescent field, in the first and second cladding layers 220, 240 (e.g., outside the quantum well 230). Accordingly, the sinusoidal structure of the surface 245 of the second cladding layer 240 provides velocity matching between the group velocity of an optical signal transmitted along the optical waveguide 220, 230, 240 and the group velocity of a microwave signal transmitted along the microwave waveguide 250, 260. As is described in detail below, velocity matching is achieved by selecting the periodicity of the sinusoidal structure of the second cladding layer 240.

Method of Determining Periodicity of Intrinsic Layer

Referring now to FIG. 3, a dispersion curve 300 of the characteristic waveguide optical modes of electro-absorption modulator 100, 200, in accordance with one embodiment of the invention, is shown. The dispersion curve 300, representing the collection of possible photonic states, plots the energy (e.g., Planck's constant times frequency) 305 versus wave number (k_(Z)) 310 in both the forward 315 and backward 320 propagating directions.

The substantially sinusoidal structure of the optical waveguide induces a photonic bandgap 325 in the propagation characteristics of supported optical waveguide modes. If the substantially sinusoidal structure of the optical waveguide has a period equal to Λ, photonic bandgaps 325 are formed wherever the forward and backward propagating states of equal energy are separated by 2π/Λ (see e.g., 330 of FIG. 3).

Additionally, the optical mode group velocity is proportional to the slope of the dispersion curve (dω/dk_(Z)). As the operating mode of the optical waveguide approaches (e.g., 335 to 340) the photonic bandgap 325, the group velocity of the optical signal decreases as the slope of the dispersion curve 300 decreases. Hence, embodiments of the invention match the optical and microwave signal velocities by selecting the periodicity 121, 221 of the sinusoidal structure of the optical waveguide such that the supported operating wave mode is at a point on the dispersion curve where the velocity of the optical signal is approximately equal to the velocity of the microwave signal.

Method of Fabricating Electro-Absorption Modulator

Referring now to FIG. 4, a flow diagram of a method of fabricating an electro-absorption modulator, in accordance with one embodiment of the invention, is shown. It is appreciated that the method provides for simultaneously fabricating many electro-absorption modulators on a single wafer. However so as not to unnecessarily obscure aspects of embodiments of the invention, fabrication of a single electro-absorption modulator is described. As depicted in FIG. 4, the method of fabrication begins with one or more initial fabrication processes on a semiconductor substrate 210.

At 410, a first cladding layer 220, a quantum well region 230, and a second cladding layer 240 are formed on the substrate 210. In accordance with one embodiment of the invention, the first cladding layer 220 is formed by depositing a semiconductor layer on the substrate 210. The first cladding layer 220 is deposited by a method such as epitaxial chemical vapor deposition (CVD). The first cladding layer 220 is composed of indium gallium arsenide phosphide (InGaAsP) or indium gallium aluminum arsenide (InGaAlAs). The first cladding layer 220 has a thickness of approximately 1-3 μm. The first cladding layer is lightly doped with an n-type impurity by introducing the n-type impurity during the chemical vapor deposition process.

In accordance with embodiments of the invention, the quantum well region 230 is formed on the surface of the first cladding layer 220. In one embodiment in accordance with the invention, the quantum well region 230 is formed by depositing a first barrier layer, a quantum well layer and a second barrier layer. In another embodiment in accordance with the invention, the quantum well region 230 is formed by alternately depositing barrier layers and quantum well layers, until N quantum layers interleaved between N+1 barrier layers are formed. The barrier layers are formed by a method such as epitaxial chemical vapor deposition. The barrier layers are composed of indium gallium phosphide (InGaP) or indium aluminum arsenide (InAlAs). The quantum well layers are formed by a method such as epitaxial chemical vapor deposition. The quantum well layers are composed of indium arsenide phosphide (InAsP) or indium gallium arsenide (InGaAs). The quantum well region 230 is deposited having a thickness of approximately 0.1-0.3 μm. It should be appreciated that the outermost barrier layers are typically thicker than the other barrier layers.

In accordance with one embodiment of the invention, the second cladding layer 240 is formed by depositing a semiconductor layer on the quantum well region 230. The second cladding layer 240 is deposited by a method such as epitaxial chemical vapor deposition (CVD). The second cladding layer 240 is composed of indium phosphide (InGaAsP) or gallium arsenide (InGaAlAs). The second cladding layer 240 has a thickness of approximately 1-3 μm. The second cladding layer 240 is lightly doped with a p-type impurity by introducing the p-type impurity during the chemical vapor deposition process.

At 420, a sinusoidal surface relief 225 is formed in the surface of the second cladding layer 240. The sinusoidal surface relief 225 of the second cladding layer 240 has a periodicity of approximately 1-500 μm, in the x-direction, and a peak-to-peak height of 10-1000 nm. The sinusoidal surface relief 225 is formed in the second cladding layer 240 by the combination of photolithography and anisotropic etching.

In one embodiment in accordance with the invention, a photoresist layer (e.g., a conventional light sensitive polymer) is spun onto the surface of the second cladding layer 240. The photoresist is exposed to a light source (e.g., ultraviolet or near-ultraviolet light) through a gray level mask. The gray level mask carries patterns with areas of different transmittances (e.g., stepped or sinusoidal varying transmittance). When the pattern is transferred to the photoresist during exposure to the light source, areas of different transmittance in the gray level mask create areas of different thickness in photoresist after development. For a negative photoresist, the molecules of the photoresist are polymerized (e.g., cross-linked) in areas exposed to the light. For a positive photoresist, molecular bonds are broken where the photoresist is illuminated. The unexposed portions of the resist remain unaffected. The unpolymerized areas of the photoresist are then selectively dissolved in a chemical wash, thereby forming a patterned photoresist. An anisotropic etching process (e.g., ion etching) is then utilized to transfer the sinusoidal profile of the patterned photoresist into the surface of the second cladding layer 240. The etchant removes the patterned photoresist and exposed portion of the second cladding layer 240 at a controlled rate such that the desired sinusoidal surface relief 225 is formed in the surface of the second cladding layer 240.

In another embodiment in accordance with the invention, a photoresist layer (e.g., a conventional light sensitive polymer) is spun onto the surface of the second cladding layer 240. The photoresist is exposed to a light source utilizing a Lloyds-mirror interferometric lithography system. The Lloyds-mirror interferometric lithography system includes a mirror fixed at an angle of 90° with respect to the x-y plane. The mirror and substrate 210 are centered about an incident beam from a light source (e.g., ultraviolet or near-ultraviolet light). Rotating the mirror and substrate 210 assembly about the center point of the incident beam varies the spatial-period of the diffraction pattern. The diffraction pattern produces sinusoidal intensities within the photoresist. Accordingly, the patterned photoresist has a sinusoidal profile. An anisotropic etching process (e.g., ion etching) is then utilized to transfer the sinusoidal profile of the patterned photoresist into the surface of the second cladding layer 240. The etchant removes the patterned photoresist and exposed portion of the second cladding layer 240 at a controlled rate such that the desired sinusoidal surface relief 225 is formed in the surface of the second cladding layer 240.

In yet another embodiment in accordance with the invention, a photoresist layer (e.g., a conventional light sensitive polymer) is spun onto the surface of the second cladding layer 240. The photoresist is exposed to a light source (e.g., ultraviolet or near-ultraviolet light) through a striped mask. The contact distance between the second cladding layer 240 and the striped mask is varied such that a diffraction pattern is obtained that produces sinusoidal intensities within the photoresist. Accordingly, the patterned photoresist has a sinusoidal profile. An anisotropic etching process (e.g., ion etching) is then utilized to transfer the sinusoidal profile of the patterned photoresist into the surface of the second cladding layer 240. The etchant removes the patterned photoresist and exposed portion of the second cladding layer 240 at a controlled rate such that the desired sinusoidal surface relief 225 is formed in the surface of the second cladding layer 240.

At 430, the first cladding layer 220, the quantum well region 230, and the second cladding layer 240 are patterned to define a ridge structure. If the surface of the second cladding layer 240 is heavily doped, the ridge structure also includes the second contact region 275. In one embodiment of the invention, the first cladding layer 220, the quantum well region 230, and the second cladding layer 240 are patterned by a method such as photolithography of a light sensitive polymer, and selectively anisotropically etching the exposed semiconductor layer. The resulting first cladding layer 220, quantum well region 230 and second cladding layer 240 form an optical waveguide structure approximately 1-4 μm wide, and 100-500 μm long.

At 440, a first and third electrodes 250, 255 and a second electrode 260 are formed. The first and third electrodes 250, 255 are formed on the substrate 210 along either side of the ridge structure. The first and third electrodes 250, 255 are approximately 50-1000 nm thick, 10-30 μm wide and 100-500 μm long. The second etelectrode 260 is formed on top of the ridge structure. The second electrode 260 is approximately 50-1000 nm thick, 2.5-10 μm wide and 100-500 μm long. The first and third electrodes 250, 255 and the second electrode 260 are formed utilizing photolithography of a light sensitive polymer to form an appropriate mask and sputtering a metal such as gold.

Prior to forming the first cladding layer 220, the quantum well region 230 and the second cladding layer 240, the substrate 210 may optionally be heavily doped with an n-type impurity to form a first contract region 270. The first contact region 270 is formed by eptaxially depositing a heavily n-doped portion of the first cladding layer 220 that is approximately 0.5-2 μm thick. The heavily doped region 270 of the substrate 210 provides a low ohmic contact for connecting first and third electrodes 250, 255 and a first cladding layer 220.

Before defining the ridge structure, the upper portion of the second cladding layer 240 may optionally be heavily doped with a p-type impurity by increasing the flow of the p-type dopant precursor during the chemical vapor deposition process of 430, to form a second contact region 275. The impurity profile extends approximately 0.5-2 μm below the surface of the second cladding layer 275. The second contact region 275 provides a low ohmic contact for connecting a second electrode 260 and the second cladding layer 240.

Referring now to FIG. 5, a flow diagram of a method of fabricating an electro-absorption modulator, in accordance with another embodiment of the invention, is shown. It is appreciated that the method provides for simultaneously fabricating many electro-absorption modulators on a single wafer. However so as not to unnecessarily obscure aspects of embodiments of the invention, fabrication of a single electro-absorption modulator is described. As depicted in FIG. 5, the method of fabrication begins with one or more initial fabrication processes on a semiconductor substrate 110. The initial fabrication processes may include cleaning, depositing, doping, etching and/or the like. The semiconductor substrate 110 is composed of indium phosphide or a gallium arsenide.

At 510, a semiconductor layer is deposited upon the substrate 110 to form the first cladding layer 120. The first cladding layer 120 is deposited by a method such as epitaxial chemical vapor deposition (CVD). The first cladding layer 120 is composed of indium phosphide (InGaAsP) or gallium arsenide (InGaAlAs). The first cladding layer 120 has a thickness of 1-3 μm. The first cladding layer is lightly doped with an n-type impurity by introducing the n-type impurity during the chemical vapor deposition process.

At 520, a sinusoidal surface relief 125 is formed in the surface of the first cladding layer 120. The sinusoidal surface relief 125 of the first cladding layer has a periodicity of approximately 1-500 μm and a peak-to-peak height of approximately 10-1000 nm, along the desired direction of propagation of an optical signal. The sinusoidal surface relief 125 is formed in the surface of the first cladding layer 120 in a manner similar to that described with regard to 420 of FIG. 4.

At 530, a quantum well region 130 and a second cladding layer 140 are conformally formed on the surface of the first cladding layer 120. The quantum well region 130 is formed on the sinusoidal surface of the first cladding layer 120. In accordance with one embodiment of the invention, the quantum well region 130 is formed by conformally depositing a first barrier layer, a quantum well layer and a second barrier layer. In accordance with another embodiment of the invention, the quantum well region 130 is formed by alternately conformally depositing barrier layers and quantum well layers, until N quantum layers interleaved between N+1 barrier layers are formed. The barrier layers are composed of indium gallium phosphide (InGaP) or indium aluminum arsenide (InAlAs). The quantum well layers are formed by a method such as epitaxial chemical vapor deposition. The quantum well layers are composed of indium arsenide phosphide (InAsP) or indium gallium arsenide (InGaAs). The quantum well region 130 is conformally deposited having a thickness of approximately 0.1-0.3 μm. The resulting conformally formed quantum well region 130 has a periodicity substantially equal to that of the surface relief formed at 520.

In accordance with one embodiment of the invention, the second cladding layer 140 is formed by conformally depositing a semiconductor layer on the quantum well region 130. The second cladding layer 140 is conformally deposited by a method such as epitaxial chemical vapor deposition (CVD). The second cladding layer 140 is composed of indium gallium arsenide phosphide (InGaAsP) or indium gallium aluminum arsenide (InGaAlAs). The second cladding layer 140 is lightly doped with a p-type impurity by introducing the p-type impurity during the chemical vapor deposition process. The second cladding layer 140 has a thickness of approximately 1-3 μm. The resulting conformally formed second cladding layer 140 has a periodicity substantially equal to that of the surface relief formed at 520.

At 540, the first cladding layer 120, the quantum well region 130, and the second cladding layer 140 are patterned to define a ridge structure. If the surface of the second cladding layer 140 is heavily doped, the ridge structure also includes the second contact region 175. In one embodiment of the invention, the first cladding layer 120, the quantum well region 130, and the second cladding layer 140 are patterned by a method such as photolithography of a light sensitive polymer, and selectively etching the exposed semiconductor layer. The resulting first cladding layer 120, quantum well region 130, and second cladding layer 140 form an optical waveguide structure approximately 1-4 μm wide, and 100-500 μm long.

At 550, a first and third electrode 150, 155 and a second electrode 160 are formed. The first and third electrodes 150, 155 are formed on the substrate 110 along either side of the ridge structure. The first and third electrodes 150, 155 are approximately 50-1000 nm thick, 10-30 μm wide and 100-500 μm long. The second electrode 160 is formed on top of the ridge structure. The second electrode 160 is approximately 50-1000 nm thick, 2.5-10 μm wide and 100-500 μm long. The first and third electrodes 150, 155 and the second electrode 160 are formed utilizing photolithography of a light sensitive polymer to form an appropriate mask and sputtering a metal such as gold.

Prior to forming the first cladding layer 120, the substrate 110 may optionally be heavily doped with an n-type impurity to form a first contract region 170. The first contact region 170 is formed by epitaxially depositing a heavily n-doped portion of the first cladding layer 120 that is approximately 0.5-2 μm thick. The heavily doped region 170 of the substrate 110 provides a low ohmic contact for connecting first and third electrodes 150, 155 and a first cladding layer 120.

The surface of the second cladding layer 140 may optionally be planarized. The substantially sinusoidal surface relief of the second cladding layer 140 may be removed by a method such as chemical mechanical polishing (CMP).

Prior to defining the ridge structure, the upper portion of the second cladding layer 140 may optionally be heavily doped with a p-type impurity by increasing the p-type dopant during the chemical vapor deposition process of 530, to form a second contact region 175. The impurity profile extends approximately 0.5-2 μm below the surface of the second cladding layer 140. The second contact region 175 provides a low ohmic contact for connecting a second electrode 160 and the second cladding layer 140.

Embodiments of the invention include an electro-absorption modulator having an optical waveguide and a microwave waveguide electro-magnetically coupled to the optical waveguide. A substantially sinusoidal structure extends along a direction of propagation of an optical signal through the optical waveguide. The substantially sinusoidal structure introduces a photonic bandgap. The group velocity of the optical signal slows as the operating mode approaches the photonic bandgap. A reduction in waveguide optical mode velocity for a given wavelength is effected by the appropriate choice of the periodicity of the substantially sinusoidal structure. Accordingly, embodiments of the invention advantageously provide for substantial velocity matching of the microwave and optical signals without large changes in the device geometry and without increased optical transmission losses.

The foregoing descriptions of specific embodiments in accordance with the invention have been presented for purposes of illustration and description. The scope of the invention is defined by the Claims appended hereto and their equivalents. 

1. An electro-absorption modulator, comprising: an optical waveguide, comprising: a quantum well region, and a substantially sinusoidal structure coupled to said quantum well region, said optical waveguide having a waveguide mode responsive to said substantially sinusoidal structure; and a microwave waveguide electromagnetically coupled to said optical waveguide, wherein said substantially sinusoidal structure has a periodicity that provides velocity matching between the group velocity of an optical signal propagating through said optical waveguide and the phase velocity of a microwave signal propagating along said microwave waveguide.
 2. The electro-absorption modulator according to claim 1, wherein: said optical waveguide additionally comprises: a first cladding layer having a sinusoidal surface relief, said sinusoidal surface relief having a first periodicity; and a second cladding layer; said quantum well region comprises a single quantum well structure between said cladding layers, said quantum well structure constituting said substantially sinusoidal structure and comprising a sinusoidal quantum well layer having a periodicity substantially equal to said first periodicity between sinusoidal barrier layers having a periodicity substantially equal to said first periodicity; and said microwave waveguide comprises: a first electrode extending alongside said first cladding layer; and a second electrode over said second cladding layer.
 3. The electro-absorption modulator according to claim 1, wherein: said optical waveguide additionally comprises: a first cladding layer; and a second cladding layer; said quantum well region comprises a multiple quantum well structure between said first cladding layer and said second cladding layer, said multiple quantum well structure constituting said substantially sinusoidal structure; and said microwave waveguide comprises: a first electrode extending along a first side of said first cladding layer; and a second electrode over said second cladding layer.
 4. The electro-absorption modulator according to claim 3, wherein: said first cladding layer has a sinusoidal surface relief, said sinusoidal surface relief having a first periodicity; and said multiple quantum well structure comprises sinusoidal quantum well layers having a periodicity substantially equal to said first periodicity interleaved with sinusoidal barrier layers having a periodicity substantially equal to said first periodicity.
 5. The electro-absorption modulator according to claim 3, wherein said microwave waveguide further comprises a third electrode extending along a second side of said first cladding layer.
 6. The electro-absorption modulator according to claim 2, wherein said first cladding layer is lightly doped with a first impurity.
 7. The electro-absorption modulator according to claim 6, further comprising: a substrate upon which said first cladding layer is located; and a first contact layer heavily doped with said first impurity located in a portion of said substrate below said first cladding layer.
 8. The electro-absorption modulator according to claim 2, wherein said second cladding layer is lightly doped with a second impurity.
 9. The electro-absorption modulator according to claim 8, further comprising a second contact layer heavily doped with said second impurity atop said second cladding layer.
 10. The electro-absorption modulator according to claim 1, wherein: said optical waveguide additionally comprises: a first cladding layer; and a second cladding layer having a substantially sinusoidal surface relief extending in said direction of propagation of said optical signal, said substantially sinusoidal surface relief constituting said sinusoidal structure; said quantum well region is located between said cladding layers; and said microwave waveguide comprises: a first electrode extending alongside said first cladding layer; and a second electrode over said second cladding layer.
 11. A method of velocity matching in an electro-absorption modulator, said method comprising: providing an optical waveguide comprising a quantum well structure, a sinusoidal structure coupled to said quantum well structure and extending in a direction of propagation of an optical signal through said optical waveguide, and a microwave waveguide extending alongside said optical waveguide and electromagnetically coupled thereto; determining an operating mode of said optical waveguide; determining a phase velocity of a microwave signal propagating through said microwave waveguide; and selecting the periodicity of said sinusoidal structure such that said operating mode is displaced from a photonic bandgap by a displacement that sets a group velocity of said optical signal substantially equal to said phase velocity of said microwave signal.
 12. The method of claim 11, wherein said selecting comprises: using a dispersion curve in which a set of energies of said optical signal is plotted against a set of wave numbers and in which a photonic bandgap exists wherever a forward and a backward propagating state of equal energy are separated by 2π/L, where L quantifies the periodicity of said sinusoidal structure; and selecting said periodicity to set said operating mode to a point on said dispersion curve where said group velocity of said optical signal matches said phase velocity of said microwave signal.
 13. A method of fabricating an electro-absorption modulator, the method comprising: forming an optical waveguide by a process, comprising: forming a first cladding layer over a substrate; forming a quantum well region over said first cladding layer; forming a second cladding layer over said quantum well region; forming a substantially sinusoidal structure on top of said second cladding layer coupled to said quantum well region and extending in a direction of propagation of an optical signal through said quantum well region, said optical waveguide having waveguide mode responsive to said substantially sinusoidal structure; patterning said first cladding layer, said quantum well region and said second cladding layer to define a ridge structure; forming a first electrode along a first side of said ridge structure; and forming a second electrode over said ridge structure.
 14. The method according to claim 13, wherein said forming said quantum well region comprises forming conformal quantum well layers interleaved with three or more conformal barrier layers over said first cladding layer.
 15. The method according to claim 13, wherein said forming said quantum well region comprises forming a conformal quantum well layer between two conformal barrier layers over said first cladding layer.
 16. The method according to claim 13, additionally comprising lightly doping said first cladding layer with a first type of impurity.
 17. The method according to claim 16, additionally comprising heavily doping a portion of said substrate adjacent said first cladding layer with said first type of impurity to form a first contact region.
 18. The method according to claim 13, additionally comprising lightly doping said second cladding layer with a second type of impurity.
 19. The method according to claim 18, additionally comprising heavily doping a portion of said second cladding layer remote from said quantum well region with said second type of impurity to form a second contact region.
 20. The method according to claim 13, further comprising forming a third electrode along a second side of said ridge structure.
 21. The method according to claim 13, wherein said forming said substantially sinusoidal structure comprises: forming a sinusoidal relief on top of said first cladding layer extending in said direction of propagation of said optical signal; conformally forming said quantum well region over said first cladding layer; and conformally forming said second cladding layer over said quantum well region.
 22. The method according to claim 21, wherein said forming said quantum well region comprises forming conformal quantum well layers interleaved with three or more conformal barrier layers.
 23. The method according to claim 21, wherein said forming said quantum well region comprises forming a conformal quantum well layers between two conformal barrier layers. 